Containers made of single ply paperboard stock have for many years been heavily coated with a wax or wax-like material so as to render the paperboard fluid-impervious, and thereby more suitable for containing foodstuffs (particularly liquids). In this regard, it has been the conventional practice to apply an excess volume of wax to the interior and/or exterior walls of the single ply paperboard container, and to thereafter drain or otherwise remove the unneeded wax from the cup. According to this conventional wax-coating technique, therefore, the applied wax saturates the entire thickness of the paperboard walls to render the container fluid-impervious. Specific examples in the art whereby single ply paperboard containers are saturated with wax include U.S. Pat. Nos. 1,175,406 and 1,197,324.
One problem associated with heavily waxed paperboard containers is that the wax is visibly perceptible on the exterior surface--i.e., since the wax saturates the entire thickness of the paperboard and transforms the normally opaque paperboard stock to an essentially translucent visual appearance. This transformation of the normal opacity of paperboard containers to translucency due to the effect of the saturated wax in turn "dulls" the otherwise vivid color graphics and/or indicia that may be printed upon the exterior surface of the paperboard container. The relatively dulled container appearance (as compared to non-wax-coated containers having the same color graphics and/or indicia) that results is less than aesthetically desirable.
One solution that has been practiced extensively in the art is to make multi-ply paperboard containers (i.e., having inner and outer paperboard plies) with a wax-barrier layer therebetween as represented by U.S. Pat. Nos. 3,450,327 and 3,603,218. When wax is applied to the inner paperboard ply according to this prior art technique, the wax-barrier layer (which is typically a layer of adhesive material that laminates the inner and outer paperboard plies one to the other) prevents the wax from penetrating to the outer paperboard ply. As a result, the outer paperboard ply retains its normal opacity, such that color graphics and/or indicia on its exterior surface of are not dulled by the presence of wax that would have otherwise occurred had the container been fabricated from single ply paperboard stock.
Recently, significantly more "glossy" polymer-coated containers having improved appearances over heavily wax coated paperboard containers have been proposed. These relatively more "glossy" containers usually are constructed of an interior layer of paperboard that is laminated on its interior and/or exterior surfaces with a suitable thermoplastic polymeric material, for example, polyethylene. In this regard, U.S. Pat. Nos. 4,168,676, 4,211,339 and 4,283,189 generally disclose paperboard containers which are electrostatically spray-coated with a thermoplastic polymer powder. The spray-coated powder on the cup surface is then subjected to heat treatment, whereby the polymeric powder melts and forms a laminated polymeric coating on the paperboard layer.
Except for appearance characteristics, wax-coated containers are preferred for a number of reasons, including lower raw material costs and/or relative ease of container recyclability, to name just a few. What has been needed in the art, therefore, are improved wax-coating methods and apparatus whereby wax-coated single ply paperboard containers are made to exhibit an aesthetically desirable "glossy" exterior surface. In such a manner, the "glossy" wax-coated containers would exhibit appearance characteristics comparable to polymer-coated paperboard containers, while yet preserving the other beneficial attributes associated with conventional wax-coated containers. It is towards providing such methods and apparatus that the present invention is directed.